Web-formed materials, such as paper or pulp, are usually dried either in a contactless manner by blowing hot air against the web-formed material, or by contact with heated surfaces, primarily cylinders.
In cylinder drying of a web-formed material, for example paper, the web-formed material is heated by heated cylinders against which the web-formed material is pressed by the web tension and/or with the aid of a felt or a dryer screen.
In contactless drying, the web-formed material is usually passed back and forth through a plurality of drying decks, floating between upper and lower blow boxes, which blow out hot process air against the web-formed material, in order to dry said material.
If the web-formed material is sufficiently porous, one useful method is to blow and/or suck process air or other suitable drying medium through the material, so-called through drying. The web-formed material is then suitably supported by a gas-permeable dryer screen or by perforated cylinders during the drying. Through drying is suitable for drying, for example, soft crepe paper (soft tissue, non-woven) and glass fibre. The concept drying is used in a broad sense in the following so that it also includes extraction of steam other than water and supply of heat for the purpose of, for example, curing a binder or achieving other chemical changes.
The water (or other substance) which, in the form of steam, leaves the web-formed material is mixed with and discharged by the process air. To be able to retain the drying effect, therefore, part of the process air must be discharged as exhaust air and be replaced by drier and preferably hot supply air. This, of course, occurs to such a limited extent that such a high moisture content is maintained in the exhaust air that condensation and corrosion on exposed parts can only just be avoided. The main part of the process air is recirculated.
The process air is heated by the supply of heat to the mixture of supply air and recirculated process air. This often takes place by recuperative heat exchange, where the heating medium is low-pressure steam or medium-pressure steam, but may also take place in other ways, for example by means of one or more gas burners placed directly in the recirculation flow. In case of an increased drying requirement, the supply of heat is increased and in case of a decreased drying requirement, the supply of heat is reduced.
In through drying, the distribution of the velocity and temperature of the process air over the surface of the web are very sensitive parameters. This is true to a particularly high degree when drying a wet-formed glass-fibre web. To ensure, as far as possible, at least a uniform velocity distribution, a perforated plate or the like is usually placed near the web-formed material on the upstream side. With this plate, a pressure drop is created which equalizes the differences in velocity to a certain extent. The higher the pressure drop, the better the equalization.
Increasing quality demands, however, have led to a situation where it is now difficult to fulfil the demands made with reasonable pressure drops.
It is a first object of the present invention to provide a through dryer for the web-formed material.
It is a second object of the present invention to provide a through dryer for a web-formed material, which dryer, with reduced pressure drop, achieves the desired conditions as regards distribution of velocity through the web-formed material.
It is a third object of the present invention to provide a through dryer for a web-formed material which fulfils higher demands as regards distribution of velocity through the web-formed material than what can be achieved using conventional technique.
It is a fourth object of the present invention to provide a through dryer for a web-formed material which permits the dried web-formed material to fulfil higher demands than what can be achieved using conventional technique.